The present invention relates to metal/fiber composite materials, and in particular, to titanium alloy matrix composites.
Pure titanium is relatively soft, weak and extremely ductile. Through additions of other elements, the base metal is converted to an engineering material having unique characteristics, including high strength and stiffness, corrosion resistance and usable ductility, coupled with low density.
Titanium is allotropic. Up to 785.degree. C., titanium atoms arrange themselves in a hexagonal close-packed crystal array called alpha phase. When titanium is heated above the transition temperature (beta transus) of 785.degree. C., the atoms rearrange into a body-centered cubic structure called beta phase. The addition of other elements to a titanium base will favor one or the other of the alpha or beta forms.
Titanium alloys are classified into three major groups depending on the phases present: alpha, beta, or a combination of the two, alpha-beta. The table below lists common titanium alloy additions. The elements which favor (stabilize) the alpha phase are termed alpha stabilizers, those which favor the beta phase are termed beta stabilizers, and those which do not show a preference for either phase, but promote one or more desirable properties are termed neutral. The alpha stabilizers raise the beta transus temperature, i.e., the temperature at which the atoms rearrange from the alpha form to the beta form, and beta stabilizers lower the beta transus temperature.
______________________________________ Titanium Alloy Additions Alpha Beta Stabilizers Stabilizers Isomorphous Eutectoid Neutral ______________________________________ Al Mo Cr Zr O V Mn Sn N Ta Fe C Nb Si Co Ni Cu H ______________________________________
In recent years, material requirements for advanced aerospace applications have increased dramatically as performance demands have escalated. As a result, mechanical properties of monolithic metallic materials such as titanium alloys often have been insufficient to meet these demands. Attempts have been made to enhance the performance of titanium by reinforcement with high strength/high stiffness filaments or fibers.
Titanium matrix composites have for quite some time exhibited enhanced stiffness properties which closely approach rule-of-mixtures (ROM) values. However, with few exceptions, both tensile and fatigue strengths are well below ROM levels and are generally very inconsistent.
These titanium composites are fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. At least four high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide. Under superplastic conditions, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus-contacting layers of matrix material bond together by a phenomenon known as diffusion bonding. Unfortunately, at the same time a reaction occurs at the fiber-matrix interfaces, giving rise to what is called a reaction zone. The compounds formed in the reaction zone may include reaction products like TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding. Titanium matrix composites have not reached their full potential, at least in part, because of problems associated with instabilities of the fiber-matrix interface. The reaction zone surrounding a filament introduces sites for easy crack initiation and propagation within the composite, which can operate in addition to existing sites introduced by the original distribution of defects in the filaments. It is well established that mechanical properties are influenced by the reaction zone, that, in general, these properties are degraded in proportion to the thickness of the reaction zone.
The cost of conventional titanium alloy foils is very high because of the many vacuum anneal/cold rolling cycles required. It is especially difficult to produce rolled foils from the alpha-beta alloy group due to their high resistance to cold deformation. Foils of the alpha-beta alloy group are particularly expensive.
It is, therefore, an object of the present invention to provide improved titanium composites.
It is another object of this invention to provide an improved method for fabricating titanium composites.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description of the invention and the appended claims.